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We report on magnetization and magnetoresistance measurements of single-crystal, melt-processed, and polycrystalline YBa2Cu3O7− samples with the purpose of disclosing the connection between the magnetic irreversibility limit and the zero resistance temperature point as a function of applied field in the above samples with very different superconducting granularities. Another goal is to find out how much the applied fields degrade the grain couplings and the electric conductivity for the differe ...

We report on magnetization and magnetoresistance measurements of single-crystal, melt-processed, and polycrystalline YBa2Cu3O7− samples with the purpose of disclosing the connection between the magnetic irreversibility limit and the zero resistance temperature point as a function of applied field in the above samples with very different superconducting granularities. Another goal is to find out how much the applied fields degrade the grain couplings and the electric conductivity for the different field-current configurations. In homogeneous superconductors, the magnetic irreversibility line is well known as a limit below which an electric current different from zero can flow without resistance. Our data for the single crystal with a weak superconducting granularity closely follow this rule. However, the results on the other samples are quite different. Normally, good-quality melt-processed YBa2Cu3O7− samples do not exhibit the signature of superconducting granularity. Nevertheless, x-ray analysis of our melt-processed sample, containing 30 wt % of the Y2BaCuO5 phase, shows considerable misalignment of the crystallite c axis, which weakens the grain couplings and hence is expected to result in superconducting granularity. Effectively, the magnetic irreversibility lines for H c axis as well as for H ab plane exhibit, in the low-field region, the usual signature of superconducting granularity. On the other hand, the zero resistance data for increasing fields along the c axis and J ab plane split away from the irreversibility line toward the lower-temperature side. Nevertheless, for H J ab plane, the zero resistance data closely follow the irreversibility line up to the highest applied fields. In the polycrystalline YBa2Cu3O7− sample, in which the grain junctions are much weaker, the effect of an increasing applied field on the grain couplings is much stronger. The zero resistance line is already split from the irreversibility line and is lower by more than 17 K at 8 kOe. Our present measurements provide good quantitative data for the discrepancy between the zero resistance and magnetic irreversibility lines. We explain these features in terms of the superconducting glass model. ...